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Developmental changes in gene expression due to hormones can help to break down genetic constraints on sex-specific evolution

All species with separate sexes face a common challenge – distinct males and females must be produced from a genome that is shared between the sexes. Historically, scientists in different disciplines have taken different approaches to understanding how sexual species solve this challenge. Evolutionary biologists have focused on measuring the extent to which males and females share genetic variance for traits under selection. Endocrinologists have focused on how sex hormones regulate the expression of these traits. Molecular geneticists have explored how patterns of gene expression differ between the sexes. But how do these perspectives fit together?

To figure this out, a team of scientists from the University of Virginia, Georgia Southern University, Virginia Tech, and the University of Texas, Arlington, studied the development of extreme sex differences in the brown anole. In this small Caribbean lizard, males and females hatch at nearly identical sizes, but gradually diverge in growth, to the point where adult males average three times the mass of adult females. By breeding hundreds of anoles under controlled laboratory conditions, the team found that the extent to which males and females share genetic variance for growth and body size decreases sharply with age. Given that each anole possesses the same genes throughout its lifetime, what could explain this change?

By comparing patterns of gene expression in the liver across development, the team found that sex differences in gene expression increase dramatically with age. These changes in expression are particularly pronounced for genes that regulate growth, and many of the patterns of gene expression that characterize males can be mimicked by treating females with testosterone. This integrative perspective suggests that a single molecular signal of sex, such as testosterone, can orchestrate the genome-wide expression of shared genes in ways that are unique to a given sex, thereby helping to break down genetic constraints arising from a shared genome.
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